Do not require NODE_BITCOIN_CASH (fix IBD)

Summary:
IBD was stuck after 'dnsseed thread exit',
broken by D669 as found by bisection.

This commit removes NODE_BITCOIN_CASH from REQUIRED_SERVICES
so that nodes without it can become connected.

Test Plan:
make check
../qa/pull-tester/rpc-tests.py -extended
clear datadir, run IBD to completion

Reviewers: #bitcoin_abc, deadalnix, schancel, kyuupichan, dagurval

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D710